Fix check for CMake not detecting struct cmsg

This check seems to have never worked and on Linux we hard coded
the check instead. Remove this hard coded assumption and use a
working check based on check_struct_has_member instead.

This is has the side-effect of port-sharing being enabled on macOS
when compiled with cmake.

Change-Id: Ia020c696f63a2a317f001c061b2ab4da69977750
Signed-off-by: Arne Schwabe <[email protected]>
Acked-by: Frank Lichtenheld <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg29448.html
Signed-off-by: Gert Doering <[email protected]>

CMakeLists.txt

@@ -131,7 +131,6 @@ if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
     set(ENABLE_FEATURE_TUN_PERSIST 1)
     set(HAVE_LINUX_TYPES_H 1)
     set(ENABLE_DCO YES)
-    set(HAVE_CMSGHDR YES)
 elseif (${CMAKE_SYSTEM_NAME} STREQUAL "FreeBSD")
     set(TARGET_FREEBSD YES)
     set(ENABLE_DCO YES)
@@ -185,14 +184,18 @@ check_symbol_exists(mlockall sys/mman.h HAVE_MLOCKALL)
 
 check_symbol_exists(sendmsg sys/socket.h HAVE_SENDMSG)
 check_symbol_exists(recvmsg sys/socket.h HAVE_RECVMSG)
-check_symbol_exists(cmsghdr sys/socket.h HAVE_CMSGHDR)
 check_symbol_exists(openlog syslog.h HAVE_OPENLOG)
 check_symbol_exists(syslog syslog.h HAVE_SYSLOG)
 check_symbol_exists(getgrnam grp.h HAVE_GETGRNAM)
 check_symbol_exists(getpwnam pwd.h HAVE_GETPWNAM)
 check_symbol_exists(getsockname sys/socket.h HAVE_GETSOCKNAME)
 check_symbol_exists(getrlimit "sys/time.h;sys/resource.h" HAVE_GETRLIMIT)
 
+# Checking for existence of structs with check_symbol_exists does not work,
+# so we use check_struct_hash_member with a member instead
+check_struct_has_member("struct cmsghdr" cmsg_len sys/socket.h HAVE_CMSGHDR)
+
+
 # Some OS (e.g. FreeBSD) need some basic headers to allow
 # including network headers
 set(NETEXTRA sys/types.h)